Transducer



H. J. LUTH TRANSDUCER Filed Aug. 1, 1960 Dec. 8, 1964 United StatesPatent O 3,160,716 TRANSDUCER Hmoid i. Luth, Hai-Syd Chemicals, lne.,Heiland, Mich. Filed Aug. l, 195i), Ser. No. 46,668 1 Claim. (Cl.i79-115.5)

This invention relates to devices utilizing a magnetic `field and moreparticularly to a conductor coil in a transducer.

It is common practice to operate a conductor coil in an air gap having astrong magnetic iield and to pass an electric current through theconductor coil causing it to move in the magnetic field, or to move thecoil in the field to induce an electrical current in the coil. In someapplications, the coil is moved from its neutral position by a momentaryapplication of au electric current and is then returned to its neutralposition by means of mechanical spring devices. Various types ofsolenoid switches, relays and transducers, such as microphones andloudspeakers operate in this manner.

More specifically and by way of example, in a moving coil orelectrodynamic loudspeaker, a voice coil is affixed to the apex of acone or diaphragm which is flexibly supported on a frame at the basediameter of the cone by an annular cloth ring called a surround, and isalso supportedv at the voice coil connection by a smaller, ilexibleannular ning called a spiden Thevoice coil is generally mounted with itsaxis at rightangles to magnetic lines of ux in a strong magnetic field,that is produced by a ring magnet and pole piece attached to the underside of the speaker frame. The surround and the spider centrally locatethe voice coil and cone and hold them in a neutral axial position aboutthe pole piece. An audio frequencyoutput from an Vamplifier is passedthrough the' voice coil and causes a force to be exerted in a directionof the axis of the coil, displacing the coil in the magnetic eld, thedisplacement varying with the magnitude of the current passing throughthe coil.

The exible nature of the supporting devices allows the coil movement andalso supplies a spring or mechanical restoring force to move the coilback to its neutral position when the amplifier frequency is at zero.The coil and diaphragm move back and forth in an axial direction due toa variation in` the amplitude of the audio signal and produce audiblesound.- Y The coil' restoring force should increase linearly with thedisplacement force in order to accurately reproduce the audio output of'the amplifier, but an actual loudspeaker system, such restoring forceis non-linear and such non-linearity increases rapidly as thedisplacement force is increased, causing serious distortion of the soundwaves and undesirable resonances, especially when transient impulses ofa high amplitude are impressed upon the voice coil. The same defect isfound in a dynamic microphone.

A primary object of this invention is to provide a new and novel voicecoil for an electrodynamic loudspeaker or other transducers that willallow them to respond to sound waves and transient impulses and transmitsaid waves and p impulses free from undesirableresonances and disortion.

Another object of this invention is to provide anonmechanical restoringforce for a moving conductor in an electro-magnetic system.

Another object of this invention is to provide a magnetic ice`reluctance path to magnetic lines of ilux in an electromagnetic systemwhich results in the linear return of a conductor that has beendisplacedin a magnetic eld to its original or neutral position, andwhich may be used in transient switching devices, remote positioningdevices,

transducers and any electromagnetic device dependent upon a springaction for zero positioning.

Further objects and advantages will become apparent from the followingdetailed description taken in connection with the accompanying drawings,in which:

FIGURE 1 is a cross-sectional elevation of an electrodynamicloudspeaker' embodying the invention.

FIGURE 2 is an enlarged view of the voice coilcross section shown inFIGURE 1.

FIGURES 3 and 4 are traces as shown by an oscilloscope comparing theoperation of applicants loudspeakers with that of la conventionalspeaker.

While this invention is susceptible to embodiments of different forms,there isV shown iu the drawings and Will herein be described in detailan embodiment of the invention With the understanding that the presentdisclosure is to be considered as an exempliiication of they principlesof the invention, and is not intended to limit the invention to theembodiment illustrated. The scope of the invention will be pointed outin the appended claims.

Referring to the drawing, a suitable supporting structure for aloudspeaker is shown, including a permanent ring magnet 4 with a plate5, having a centrally located hole 6 therein, mounted on itsV topsurface, and a circular plate 7 with a centrally disposed cylindricalpole-piece 8, mounted on the bottom surface of magnet 4. The platesare'secured to ring magnet 4Aby a plurality of tie bolts 9. The polepiece S is smaller in diameter at its top portion than the hole 6 andextends axially upward through the ring magnet -4 and the hole 6terminatnig near the top surface of the plate 5. A strong magnetic fieldis established by the'magnet- 4 in a circular air gap formed between thesurface of hole 6 and the outer surface of pole piece 8. A plurality ofupwardly and outwardly extending legs 10 originate at the outerperiphery of plate 5 and terminate in an annular ring 11 to form thebasket of the loudspeaker and a felt ring 11a is cemented to the topsurface of ring 1l providing a cushion for mounting the loudspeaker.

The operating or moving lcomponents ot applicants 'Ilhe means lforsupporting the diaphragm and coil` consist of .an annular ring orsurround 15 of cloth or other ilexible .material forming a flexibleconnection between the basket rim 11 and the base portion of thediaphragm l2. Additional support for the diaphragm and coil is providedIby an annular ring or pider 16 of flexible sheet material atlxed to thevoice coil adjacent its connection to the apex of the diaphragm and tothe top plate 5, forming a exible connection therebetween.

The spider 16 which supports the voice coil 13 in Vthe speaker Aframe ismade more tlexible in applicants loudspeaker than in a conventionalloudspeaker for the purpose of utilizing the suspension device ascentering means only, to keep the coil in axial alignment as it is movedin the air gap, and to afford very little, if any, opposition orrestoring force to the movement of the coil.

A preferred form of applicants voice coil is shown in FIGURE 2 andcomprises a cylindrical aluminum tube 2i? having its outer ysurfaceprecoatedY with a silicone rubber 21. A cylindrical paper form 22 havinga band of magnetic material 23 disposed on its outer surface extendingfrom the edge to the center of the form is cemented concentrically aboutthe tube 20, and a conductor 24 is wound in a double Alayer coil overthe magnetic material. The ends ofthe conductor 24 are extended out toone of the legs and terminate in a connection 25.

In operation, the magnetic material 23 of the coil 13 offers arelatively low reluctance path for the magnetic lines of ilux in the airgap and as the coil is displaced axially upward or downward from itsneutral position in the air gap, by an audio signal applied to the voicecoil, Vthe magnetic tiux will follow the path of low reluctance offeredby the magnetic material rather than the path of high reluctance oeredby the air gap. YAs the `magnetic lines of flux fol-low the magneticmaterial, they Iare bent in the direction of coil movement and act torestore the coil to its neutral position when the displacement 'force isremoved or reduced to zero. The ilux lines act as a magnetic springproducing a coil restoring force that is'completely linear to the'coildisplacing force.

Various methods of coil construction may be resorted to in order toproduce a magnetic material spacially coexistent with a conductor coil,and a few are enumerated in the following examples:

Example l Example I1 A self-supporting double layer cylindrical coiledconductor is cemented to the outside surface of a cylindrical aluminumtube and the outside surface of the coil coated with a catalyzed epoxyresin, dusted with iron powder and allowed to cure. Y

Example III Y An adhesive cement is made up with a low viscosity epoxyresin and iron powder, and used-for yfastening the turns .of a two-layercylindrical conductor l'coiltogether as Well as to cement the coil tothe outer surface of a cylindrical paper form.

Example l V A strip of paper coated on one side with a Iband ofcatalyzed resin has iron powder dusted onto and rolled into the band, aswith a rubber roller, and allowed to cure. The paper is 'cemented to theouter surface of a cylindrical aluminum tube onto which a thin layer ofsilicone rubber has been precoated. A dou-ble layer of conductor coil isWound about the magnetic band. The rubber coating acts as a yieldablesurface between the tube and the conductor winding.

Example V A coil as in Example IV except that a double band of magneticmaterial is used, one on each side of the paper strip.

Example VI A magnetic iron conductor'is wound about the outer surface ofa cylindrical paper form.

Example VII A lay-er of magnetic iron foi-l is formed about and aixed tothe outer surface of a cylindrical paper form v and a conductor woundabout the iron foil.

The above examples show that the voice coil of ap plicants invention canrbe made up in -many various ways. It should be noted that the greaterthe amount of magnetic material used in the construction of the coil,the lower the reluctance to the magnetic line of ilux.

The curves of FIGURES 3 and 4 show a comparison between the restoringforces operating on a loudspeaker coil having a magnetic materialdisposed coaxially with the coil conductor as described -in Example Vand an identical speaker coil but without the magnetic material, asmeasured on an oscilloscope. In each instance, the coils were placed ona magnetic structure and fastened with a spider except that the spiderused with the magnetic coil was of a larger diameter in order to form amore liexible Imounting and relegato its function to centering purposesonly. The coils were raised a fixed distance aboverthe gap and released.It can be said that if there werel no inductance in the coil, and noinitial forces or gravitational forces that the trace would be .astraight line. The trace shown in FIGURE 3 is the result obtained fromthe magnetic coil and it can be seen that the restoring force iscompletely linear. In FIGURE 4, the trace obtained by the non-magneticcoil shows a'slow take-off which is due to the restriction offered lbythe spider and a non-linear restoring force -where considerable bouncingtook placeldue to the spiden It is apparent trom viewing the two curvesthat Vthe restoring force offered by the magnetic spring effect issuperior to the mechanical spider device since the restoring force ofthe magneticspring is completely Y linear to the coil displacing force.

Utilizing applicants invention in a loudspeaker results in excellenttime constants when transient impulses are imposed on the voice coil.Because of the linear restoring force provided by the magnetic springeffect, ap plications for applicants invention include, any `devicedependent upon lspring Iaction for zero positioning, such as meters,relays, transient switching devices and remote positioning devices.

I claim:

' A transducer comprising: a voice coil consisting of a cylindricalnon-magnetic form with a current conductor wound about its outer surfaceand having a magnetic materialand a thin layer of yieldable materialinterposed between said conductor and said outer surfacega cylindricalpole piece having lines of magnetic ilux extending radially outwardtherefrom; means for mounting said voice coil concentric with and aboutsaid pole piece so that said coil is free to move in its axial directionfrom a 5 5 Y n neutral position; means for moving said voice coil in a2,522,859 9/50 Carbonneau 179-1155 direction at right angles to saidlines of magnetic iux; and 2,53 8,691 1/ 51 Kim 179-1155 means forbending said ux lines in the direction of coil 2,590,554 3/52 Lukacs179-1155 movement including `a low reluctance path to said flux2,352,059 11/58 Marchand et al 179-1155 lines oered by said magneticmaterial and which serves 5 FOREIGN PATENTS 1zlintlhte principal force'to return the co11 to its neutral posr- 447,846 5/36 Great Britain.

119,719 5 30 Austria.

References Cited by the Examiner 253159 6/26 Great Britain 4 UNTTEDSTATES PATENTS lo ROBERT H. ROSE, Primary Examiner.

1,921,924 8/33 Hunter 179-1155 L. MILLER ANDRUS,WILL1AM C. COOPER,1,962,012 6/34 Grossmann 179-1155 Examiners.

